Stitching refers to the alignment of the print from multiple jetting modules for the purpose of creating the appearance of a single page-width line head. For example, as shown in FIG. 1, seven jetting modules 2—each three inches in length—can be stitched together at junctures 2a to form a twenty-one inch page-width line head 4. The page-width image data is processed and segmented into separate segments for each jetting module 2, and then a segment is sent—with an appropriate module-to-module delay to account for the staggered separation of the modules—to the print nozzles 6 of each jetting module for printing.
However, though it may be anticipated that the module-to-module alignment may be very good, mechanical tolerances may be difficult to consistently maintain and alignment will often not be perfect. Moreover, even if the jetting modules are perfectly aligned, differences in the nozzle aim between jetting modules may make them appear to be misaligned in the printed output. Consequently, this type of conventional, multi-segment jetting module configuration suffers from the drawback that the pitch of the output lines along the juncture 2a of adjacent jetting modules is irregular and thereby causes lines of lower (if too far apart) or higher (if too close together) density to appear at the juncture 2a of each jetting module segment and thus impairs the quality of the printed pattern of the output. In the output medium, such misalignment typically produces a gap or “white line” artifact 8a (as shown in FIG. 2A) or an overlap or “dark line” artifact 8b (as shown in FIG. 2B).
With a view to overcoming the presence of visible gaps or bands in the printed image, U.S. Pat. No. 7,118,188 deliberately sets the print dies of an inkjet printer with a small overlap, specifically no more than a few times the nozzle spacing. As a result of the redundancy of nozzles in the region where adjacent dies overlap, this gives flexibility for compensating for gaps or bands produced by inaccuracies in locating the dies and thus in setting the overlap dimension. Although, in an ideal case, 100% of the required amount of ink (maximum) would be printed by only 50% of the nozzles of each die in the overlap region, in practice more or fewer of the nozzles may be fired to compensate for imperfections. For example, if the overlap is less than intended, the production of a gap is avoided by firing some of the nozzles which would not be fired in the ideal case.
A printing mask is a means for selectively masking off certain nozzles, i.e., preventing these nozzles from firing even if printing instructions for those nozzles should include an instruction to fire. U.S. Pat. No. 7,118,188 further discloses a method of adding stitching masks to the printed image content, where artifacts in the printed image caused by the printing nozzles in the overlapping region are removed, either by (a) measuring the width of the band produced in the overlapping region and selecting an appropriate stitching mask for subsequent printing operations, or by (b) printing out a test pattern in which areas corresponding to a range of stitching masks are printed out and the optimal mask is selected for subsequent printing operations. The stitching mask is then added to, or superimposed on, the printing masks to ensure that the required correction is made independently of the content to be printed.
U.S. Pat. No. 7,118,188 further discloses that the target may comprise a array of target patches overlapping the die-to-die boundaries and including a range of stitching masks. The magnitudes of the die-to-die boundary artifacts are then assessed either by a user of the machine or automatically by an optical sensor/scanner system. In the first option, a user visually examines the patches in each row and selects the one with the better area fill uniformity at the printed region corresponding to the die-to-die boundary. The corresponding stitching mask is then applied to that die-to-die boundary in subsequent normal printing operations. In the second option, an optical sensor moves over all the patches detecting the boundary artifact level and supplies the most appropriate stitching mask for each die pair to a printer control system, where the masks will then be used in subsequent normal printing operations.
In relation to page-width thermal printers, U.S. Pat. Nos. 4,977,410 and 5,450,099 each disclose a thermal line printer including a plurality of staggered linear head segments arranged in a pair of parallel rows such that the head segments partly overlap with each other in overlapping regions near the ends of each segment. In U.S. Pat. No. 5,450,099, the print data in the overlapping region is interleaved to eliminate boundary artifacts at the juncture between segments. In U.S. Pat. No. 4,977,410, the initial assignment of image bit data to a segment in the overlapping region is shifted lengthwise to accommodate for boundary artifacts at the juncture between segments.
In relation to a carriage-type printer wherein a printhead is attached to a carriage that is reciprocated to print one swath of information at a time on a stationary receiving medium, U.S. Pat. No. 6,663,206 discloses methods for masking stitch errors between adjacent swaths laid down by operation of such a printer. In contrast with the afore-mentioned examples of page-width printers that utilizes a line head including an array of stationary printheads, after each swath is printed by the carriage-type printer the receiving medium is stepped a distance equal to the height of the swath so that the next printed swath overlaps the pixels from the last line of the previously printed swath. When a controller determines that a stitch joint error will occur based on the current relative location between the printhead and the medium and the location of the previous swath, the location of the next swath is adjusted relative to the position of the previous swath to eliminate the stitch joint error.
According to U.S. Pat. No. 6,663,206, the data is shifted in the printhead so that the data for the next swath is aligned within a predetermined pixel accuracy to the measured paper position, e.g., by having a later nozzle fire the pixel data originally set to be fired by the first nozzle of the printhead. In addition, the remaining stitch joint error is covered up by modifying the pixels at the stitch interface. In one example, the pixels created in the region between the last line of the previous swath and the first line of the next swath can be a duplicate line of either the last line of the previous swath or the first line of the next swath, where the size and/or density of the pixels can be changed. In another example, for situations where the stitch error is less than a pixel, in addition to shifting the data and firing the information set to be printed, the controller will also fire a line of fill pixels from the nozzle prior to and immediately adjacent to the first-fired nozzle. The purpose of a fill pixel is to bridge the gap between a printed pixel from the last fired nozzle of the previous swath and a corresponding adjacent printer pixel that will be formed when the first line of pixels is formed by the nozzle that will be used for the first line of pixels for the next swath. According to U.S. Pat. No. 6,663,206, the fill pixels create a printed image having more uniform continuity and density. The fill pixels are not produced for all of the pixels located in the last line of the previous swath. Instead, the fill pixels are produced when a printed pixel is located in the same position in both the previous swath and the next swath. The fill pixels can also be at a reduced size and/or density.
As thus understood in the prior art, stitch joint error in a drop-on-demand carriage-type system can be the result of a gap between the drop of one swath adjacent the stitch joint and the drop of an adjoining swath adjacent the same stitch joint. As explained in U.S. Pat. No. 6,663,206, the gap is usually caused by difficulties in producing adjacent swaths close enough together to mask this apparent error, and the correction must be produced on-the-fly during a production run. In contrast, as also explained in the '206 patent, a page-width printer includes a stationary printhead having a length sufficient to print across the width or length of the sheet of receiving medium. The receiving medium is continually moved past the page-width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. Thus, it would be understood that a page-width printer would avoid the need for on-the-fly corrections between swaths during a production run.